Refactor FeatureBitset:

* Remove composite helper functions
* Add set difference and Bitset/uint256 operators
* Convert tests to use new feature bitset set difference operator

src/test/app/Path_test.cpp

@@ -1025,7 +1025,8 @@ public:
     {
         testcase("Path Find: CNY");
         using namespace jtx;
-        Env env{*this, supported_features_except(featureFlow)};
+        Env env{*this,
+                supported_amendments().reset(featureFlow)};
 
         Account A1 {"A1"};
         Account A2 {"A2"};